When the Nets overhauled their roster last summer, they were supposed to be going from a team that couldn’t get past an injury-riddled Bulls team in the first round a year ago to one that would be a serious contender for an Eastern Conference crown and beyond.

Instead, after the Nets saw their massive fourth quarter comeback come up short and lose 115-113 to the Raptors in front of a sellout crowd of 20,393 inside Air Canada Centre Wednesday, they are faced with the reality of a 3-2 deficit in this best-of-seven series, and the potential for a second straight playoff exit on their home floor if they fail to win Game 6 on Friday in Brooklyn.

“We have to,” said Deron Williams when asked if the Nets could save their season and get two straight wins. “We understand what the task is, and we feel like we’re up to it.”

After a horrid opening three quarters, it appeared the Nets were going to have a chance to pull off what would have been a miraculous comeback after trailing by as many as 26 points in the second half and 22 at the start of the fourth quarter when Joe Johnson’s 3-pointer tied the game at 101 with 3:19 remaining capped a 32-10 run, for the Nets to open the fourth. Kevin Garnett and Paul Pierce missed the run, as they did not play in the fourth quarter.

“We dug ourselves such a big hole,” Johnson said, “and then in the second half, we finally decided that we wanted to play.”

Both teams then reeled off matching five-point bursts, and then Kyle Lowry — who starred for Toronto, finishing with 36 points and six assists while dominating his individual matchup with Williams — hit a 3-pointer and a difficult runner in the lane to give Toronto a 111-108 lead with 27 seconds left.

After Johnson, who led the Nets with 30 points, missed a runner in the lane and DeMar DeRozan subsequently hit a pair of free throws to give the Raptors a 113-108 lead with 17.5 seconds left, it looked like the Nets’ comeback was destined to fall short. But Toronto ensured there would be at least one more twist when Amir Johnson fouled Alan Anderson as he made a 3-pointer with 9.7 seconds left.

DeRozan then made a second pair of free throws with 6.5 seconds left to make it 115-112, and it again appeared Toronto had dodged a bullet when they immediately — and smartly — fouled Blatche after catching the inbounds pass with 4.9 seconds left, and before the Nets could get up a potential game-tying 3-pointer.

But after Blatche made the first one, he missed the second long — later he said he actually was trying to make it — Shaun Livingston tipped the rebound up in the air, while appearing to get fouled in the process, and Blatche came up with it.

He then tried to pass the ball to Williams at the top of the key for a potential game-winning 3-pointer, but the pass sailed far over his head and into the backcourt, causing a backcourt violation and after Toronto completed one more inbounds pass, ensured the Nets would come up short.

“I tried to make a great play to D-Will,” Blatche said, “so he can make the three and we can go home with a 3-2 lead in the series.”

Instead, the Nets go home with their season on the brink after their spirited comeback came up short, mainly because they were so horrible during the first three quarters that they left themselves such a massive hole to dig out of in the first place.

After taking a 40-36 lead midway through the second quarter, the Nets were outscored 26-6 over the final 5:13 of the first half — a run that was capped by a Lowry running 3-pointer that banked in at the buzzer — that gave the Raptors a 62-44 halftime lead, and Toronto eventually led by as many as 26 in the third quarter.

“We couldn’t get a stop,” Williams said in explaining the Nets’ ineptitude in the middle portion of the game. “We couldn’t get a stop. They were hitting some tough shots, they were the aggressor, and just were playing better than us right there.”

Toronto’s 22-point edge after three quarters turned out to be just enough for the Nets to be unable to overcome in the fourth, and now their season — one that held so much potential a few months ago when it began, and even two weeks ago when the regular season ended — is one loss away from a sudden ending.